1. Field of the Invention
The present invention relates to an image forming apparatus that includes a developing device including first and second chambers, which are vertically disposed and accommodate a developer, and controls a toner supply amount of the developing device based on an output obtained by causing a magnetic detection element to detect the developer circulating inside the developing device. More particularly, the present invention relates to toner supply control of correcting an error of a relation between the output of a magnetic detection element and an actual toner density at a predetermined time after activation of the developing device.
2. Description of the Related Art
Image forming apparatuses have widely been used in which an electrostatic image formed in an image bearing member is developed into a toner image by a developing device, the toner image of the image bearing member is transferred to a recording material directly or through an intermediate transfer member, the recording material to which the toner image is transferred is heated and pressurized, and an image is fixed to the recording material. As the developing device, a developing device of a two-component developing system is widely used which develops an electrostatic image of the image bearing member into a toner image using a two-component developer in which toner and a carrier are mixed.
The developing device of the two-component developing system will be described with reference to FIG. 2. In the developing device 1 of the two-component developing system, only the toner is extracted from the developer with formation of an image. The supply developer having toner as a main component is supplied from a supplying unit 20 to the developing device 1. In the developing device 1, a magnetic detection element 14 that measures a toner density is provided to balance consumption and supply of the toner in the developing device 1 and maintain the toner density of the developer inside the developing device 1 in a predetermined range.
The toner density is a weight ratio of the toner occupying the developer in a unit weight in the developing device 1. A suitable toner density is considered to be normally in the range of about 5% to 11%. When the toner density falls by 5%, sufficient toner may not be attached to an electrostatic image and an image density tends to decrease. When the toner density exceeds 11%, the toner that is not attached to the surface of a carrier increases. Therefore, the toner flying from a rotating developer bearing member 8 tends to increase.
Japanese Patent Laid-Open No. 10-307434 discloses an image forming apparatus that controls toner supply from the supplying unit 20 to the developing device 1 such that an estimated value of the toner density estimated based on output of the magnetic detection element 14 is maintained constantly. Here, a time at which the output of the magnetic detection element 14 is stabilized is retarded with deterioration in fluidity of the developer. Therefore, as the cumulative use time of the developer is longer, an acquisition time of the output of the magnetic detection element 14 is more delayed.
In the developing device disclosed in Japanese Patent Application Laid-Open No. 10-307434, an image starts to be formed after the circulation of the developer inside the developing device is normalized and the output of the magnetic detection element 14 is stabilized. Further, after the first image is completely formed, the supply amount of developer is determined based on the output of the magnetic detection element 14. At this time, in a vertical agitating type developing device in which a developing chamber and an agitating chamber are vertically disposed as illustrated in FIG. 2, it takes some time until the circulation balance enters a normal state after the activation of the developing device in order to realize a circulation in which the developer is raised and circulated in the developing device. For this reason, as illustrated in FIG. 5, the output of the magnetic detection element 14 may change after the activation of the developing device. In this case, even when the developer is a new product, it takes three seconds or more to start forming the first image after the activation of the developing device. When the developer becomes old and the fluidity deteriorates, it takes five seconds or more to start forming the first image after the activation of the developing device.
Therefore, the first sheet of image may not start to be formed unless an idle operation of the developing device 1 continues for three seconds or more until the stabilization of the output of the magnetic detection element 14 after the activation of the developing device. The supply amount of developer may not be determined based on the output of the magnetic detection element 14 until the first sheet of image is completely formed.
In consideration of productivity and responsiveness of the image forming apparatus and agitating deterioration of the developer in the developing device, the idle operation time after the activation of the developing device is preferably shortened as much as possible. For example, for the purpose of FAX reception of a multifunctional apparatus, an image forming process of forming only one sheet is performed in many cases. It is assumed that an image forming process of forming one sheet is intermittently instructed ten times and the developing device is stopped/activated at every time. Then, when an image can start to be formed without waiting the stabilization of the output of the magnetic detection element, image formation productivity can be considerably improved. When the supply amount of developer is determined without waiting the stabilization of the output of the magnetic detection element 14 and the determined amount of developer is supplied at the time of forming the first sheet of image, it is not necessary to perform the idle operation to supply the developer after the formation of the first sheet of image.